It should provide you a link to your storage settings which shows how much you're using, and for which libraries if you're using storage for both personal and group libraries.
Sorry for reviving this old thread. Since I am new to Zoero, I was also wondering about this. A suggestion for the desktop/tablet/mobile app: A view to see how much cloud storage has been used would be really useful within those apps without having to open the browser.
Upgrade Storage
https://www.zotero.org/settings/storage
A suggestion for the desktop/tablet/mobile app: A view to see how much cloud storage has been used would be really useful within those apps without having to open the browser.
https://www.zotero.org/settings/storage